I am having trouble with too much blank space between colors on top of the size. The colors initially spread, but don't really touch each other. After a couple of minutes, the colors pull away from each other even more. I am using Jacquard Marbling colors. Any ideas what I could do to get better color saturation?

This sounds a bit like skin forming on top of your size. You need to skim the skin off each time you do the next round of marbling. When that skin forms it gets pushed between the colors and separates them. Let me know if this helps.